Multireference Study of the H₂COO (Criegee Intermediate) + O₃ Addition: A Reaction of Possible Tropospheric Interest

The addition of carbonyl oxides to ozone could have an effect on the tropospheric HO^• nocturnal formation. Its mechanistic description has provided so far conflicting results. CASPT2 (and CASSCF) geometry optimizations, focused on the initial addition step, show that the most likely pathway is not a concerted cycloaddition to give directly a c-H₂CO₅ intermediate, as had been proposed. Concerted cycloaddition is in fact related to a second-order saddle point that puts in turn into relation, obliquely, two distinct diradicaloid transition structures. While O-O addition goes through an energy demanding pathway, C-O addition, potentially producing the open-chain ^•OOCH₂-OOO^• diradical intermediate, corresponds to a more viable pathway. CASPT2 and MRCI optimizations, though predicting an addition diradicaloid pathway, describe ^•OOCH₂-OOO^• as an unstable structure, because fragmentation to ^•OOCH₂-O^• + O₂ takes place past the addition TS without any energy barrier: ring closure to c-H₂CO₅ is thus prevented. The addition E barrier drops as the methods become more demanding, with CASPT3//PT2 at 12.8 kcal mol^-1, MRCI at 9.1 kcal mol^-1, and MRCC at 4.8 kcal mol^-1 (possibly suggesting a significant role of dynamical correlation). Recent experimental data by Chang et al. qualify this addition as a fairly fast reaction.